IRC, #hurd, 2010-10-05

<sdschulze> antrik: Erlang-style parallelism might actually be interesting
  for Hurd translators.
<sdschulze> There are certain similarities between Erlang's message boxes
  and Mach ports.
<sdschulze> The problem is that all languages that implement the Erlang
  actor model are VM-based.
<antrik> sdschulze: I guess that's because most systems don't offer this
  kind of message passing functionality out of the box... perhaps on Hurd
  it would be possible to implement an Erlang-like language natively?
<sdschulze> That would be quite attractive -- having the same API for
  in-process parallelism and IPC.
<sdschulze> But I don't see why Erlang needs a VM...  It could also be
  implemented in a library.
<sdschulze> BTW, Scala doesn't require a VM by design.  Its Erlang
  implementation is a binary-compatible abstraction to Java.
<sdschulze> My point was that Erlang employs some ideas that might be
  usable in the Hurd libraries.
<sdschulze> concerning multithreading stuff
<sdschulze> Unfortunately, it will not contribute to readability if done in
<antrik> perhaps it's worth a look :-)
<sdschulze> Actually, a Mach port is pretty close to an Erlang actor.
<sdschulze> Currently, your I/O callbacks have to block when they're
  waiting for something.
<sdschulze> What they should do is save the Mach port and respond as soon
  as they can.
<sdschulze> So there should be a return status for "call me later, when I
  tell you to" in the callbacks.
<sdschulze> Then the translator associates the Mach port with the summary
  of the request in some data structure.
<sdschulze> As soon as the data is there, it tells the callback function to
  appear again and fulfills the request.
<sdschulze> That's -- very roughly -- my idea.
<sdschulze> Actually, this eliminates the need for multithreading
<antrik> sdschulze: not sure whether you are talking about RPC level or
  libc level here...
<sdschulze> It should be transparent to libc.
<sdschulze> If the client does a read() that cannot be answered immediatly,
  it blocks.
<sdschulze> The difference is that there is no corresponding blocking
  thread in the translator.
<antrik> ah, so you are talking about the server side only
<sdschulze> yes
<antrik> you mean the callback functions provided by the translator
  implementation should return ASAP, and then the dispatcher would call
  them again somehow
<sdschulze> allowing the server to be single-threaded, if desired
<sdschulze> exactly
<sdschulze> like: call_again (mach_port);
<antrik> but if the functions give up control, how does the dispatcher know
  when they are ready to be activated again? or does it just poll?
<sdschulze> The translator knows this.
<sdschulze> hm...
<antrik> well, we are talking about the internal design of the translator,
<antrik> I'm not saying it's impossible... but it's a bit tricky
<antrik> essentially, the callbacks would have to tell the dispatcher,
  "call me again when there is an incoming message on this port"
<sdschulze> Say we have a filesystem translator.
<antrik> (or rather, it probably should actually call a *different*
  callback when this happens)
<sdschulze> The client does a "read(...)".
<sdschulze> => A callback is called in the translator.
<antrik> let's call it disfs_S_io_read() ;-)
<antrik> err... diskfs
<sdschulze> The callback returns: SPECIAL_CALL_ME_LATER.
<sdschulze> yes, exactly that :)
<sdschulze> But before, it saves the position to be read in its internal
  data structure.
<sdschulze> (a sorted tree, whatever)
<sdschulze> The main loop steps through the data structure, doing a read()
  on the underlying translator (might be the disk partition).
<sdschulze> "Ah, gotcha, this is what the client with Mach port number 1234
  wanted!  Call his callback again!"
<sdschulze> Then we're back in diskfs_S_io_read() and supply the data.
<antrik> so you want to move part of the handling into the main loop? while
  I'm not fundamentally opposed to that, I'm not sure whether the
  dispatcher/callback approach used by MIG makes much sense at all in this
<antrik> my point is that this probably can be generalised. blocking
  operations (I/O or other) usually wait for a reply message on a port --
  in this case the port for the underlying store
<antrik> so the main loop would just need to wait for a reply message on
  the port, without really knowing what it means
<sdschulze> on what port?
<antrik> so disfs_S_io_read() would send a request message to the store;
  then it would return to the dispatcher, informing it to call
  diskfs_S_io_read_finish() or something like that when there is a message
  on the reply port
<antrik> main loop would add the reply port to the listening port bucket
<antrik> and as soon as the store provides the reply message, the
  dispatcher would then call diskfs_S_io_read_finish() with the reply
<sdschulze> yes
<antrik> this might actually be doable without changes to MIG, and with
  fairly small changes to libports... though libdiskfs etc. would probably
  need major rewrites
<sdschulze> What made me think about it is that Mach port communication
  doesn't block per se.
<antrik> all this is however ignoring the problem I mentioned yesterdays:
  we need to handle page faults as well...
<sdschulze> It's MIG and POSIX that block.
<sdschulze> What about page faults?
<antrik> when the translator has some data mapped, instead of doing
  explicit I/O, blocking can occur on normal memory access
<sdschulze> antrik: Well, I've only been talking about the server side so
<antrik> sdschulze: this *is* the server side
<antrik> sdschulze: a filesystem translator can map the underlying store
  for example
<antrik> (in fact that's what the ext2 translator does... which is why we
  had this 2G partition limit)
<sdschulze> antrik: Ah, OK, so in other words, there are requests that it
  can answer immediatly and others that it can't?
<antrik> that's not the issue. the issue is the the ext2 translator doesn't
  issue explicit blocking io_read() operations on the underlying
  store. instead, it just copies some of it's own address space from or to
  the client; and if the page is not in physical memory, blocking occurs
  during the copy
<antrik> so essentially we would need a way to return control to the
  dispatcher when a page fault occurs
<sdschulze> antrik: Ah, so MIG will find the translator unresponsive?  (and
  then do what?)
<antrik> sdschulze: again, this is not really a MIG thing. the main loop is
  *not* in MIG -- it's provided by the tranlator, usually through libports
<sdschulze> OK, but as Mach IPC is asynchronous, a temporarily unresponsive
  translator won't cause any severe harm?
<sdschulze> antrik: "Easy" solution: use a defined number of worker
<antrik> sdschulze: well, for most translators it doesn't do any harm if
  they block. but if we want to accept that, there is no point in doing
  this continuation stuff at all -- we could just use a single-threaded
  implementation :-)


<sdschulze> Hard solution: do use explicit I/O and invent a
  read_no_pagefault() call.
<antrik> not sure what you mean exactly. what I would consider is something
  like an exception handler around the copy code
<antrik> so if an exception occurs during the copy, control is returned to
  the dispatcher; and once the pager informs us that the memory is
  available, the copy is restarted. but this is not exacly simple...
<sdschulze> antrik: Ah, right.  If the read() blocks, you haven't gained
  anything over blocking callbacks.
* sdschulze adopted an ML coding style for his C coding...
<sdschulze> antrik: Regarding it on the Mach level, all you want to do is
  some communication on some ports.
<sdschulze> antrik: Only Unix's blocking I/O makes you want to use threads.
<sdschulze> Unless you have a multicore CPU, there's no good reason why you
  would *ever* want multithreading.
<sdschulze> (except poor software design)
<sdschulze> antrik: Is there a reason why not to use io_read?
<antrik> sdschulze: I totally agree about multithreading...
<antrik> as for not using io_read(): some things are easier and/or more
  efficient with mapping
<antrik> the Mach VM is really the most central part of Mach, and it's
  greatest innovation...
<sdschulze> antrik: If you used explicit I/O, it would at least shift the
  problem somewhere else...
<antrik> sure... but that's a workaround, not a solution
<sdschulze> I'm not sure how to deal with page faults then -- I know too
  little about the Hurd's internal design.
<sdschulze> Non-blocking io_read only works if we address the client side,
  too, BTW.
<sdschulze> which would be quite ugly in C IMHO
<sdschulze> announce_read (what, to, read, when_ready_callback);
<antrik> sdschulze: POSIX knows non-blocking I/O
<antrik> never checked how it works though
<sdschulze> Yes, but I doubt it does what we want.
<antrik> anyways, it's not too hard to do non-blocking io_read(). the
  problem is that then you have to use MIG stubs directly, not the libc
<sdschulze> And you somehow need to get the answer.
<sdschulze> resp. get to know when it's ready
<antrik> the Hurd actually comes with a io_request.defs and io_reply.defs
  by default. you just need to use them.
<sdschulze> oh, ok
<antrik> (instead of the usual io.defs, which does a blocking send/receive
  in one step)
<sdschulze> I'd be interested how this works in Linux...
<antrik> what exactly?
<sdschulze> simultaneous requests on one FS
<antrik> ah, you mean the internal threading model of Linux? no idea
<sdschulze> if it uses threading at all
<antrik> youpi probably knows... and some others might as well
<sdschulze> Callbacks are still ugly...